Ceramic slip composition and method for making the same

ABSTRACT

A uniform suspension of ceramic powder and method for making the same. The suspension is prepared by mixing finely divided ceramic powder in an aqueous carrier fluid, combining with a dispersing agent, and alternatively, an organic binder when forming a slip. The ceramic powder has an average particle size of about 0.5 micron or less and is present in the suspension in a loading of up to 30% by volume of the total solids in suspension. A passivating agent is present in the carrier fluid in an amount of 0.5 to 5% by weight of the ceramic powder present for suspension and slip respectively. After the addition of a dispersant, the suspension has a Bingham yield point of less than 230 dynes/cm 2  and an apparent viscosity of less than 3000 cps. A green layer produced from the slip exhibits a pore size of less than 0.5 micron.

FIELD OF THE INVENTION

[0001] The present invention pertains to improved aqueous ceramicsuspensions, slips, and ceramic green bodies and methods for producingthe same. In particular, the present invention pertains to the use of anoptimal range of passivating and dispersing agents in an aqueoussuspension of finely divided ceramic powder.

BACKGROUND OF THE INVENTION

[0002] The importance of being able to produce thinner dielectric layersis becoming increasingly recognized by the producers of multilayercapacitors (MLC's) due to end user requirements of reduced size andcost. These capacitors are typically manufactured by co-firing, i.e.,sintering a ceramic dielectric formulation and a conductive electrodematerial in an oxidizing atmosphere at a temperature in the range ofabout 1000° C. to 1400°C.

[0003] Dielectric layers have traditionally been produced by preparing asuspension of ceramic powder in a liquid vehicle, usually containing adispersant, and then adding an organic resin matrix which functions tobind the ceramic particles within the suspension. A variety of known forapplying the suspension and binder mixture (hereafter defined as slip)to a substrate to form very thin layers of the suspended solids. Methodssuch as wet coating, tape-casting (casting), or doctor-blading arereadily known to those skilled in the art. The thin, dried layersgenerally termed as green layers, may then be coated with conductiveelectrodes and stacked together with similar layers to form a greenbody. The stack is then trimmed and co-fired to produce a structureconsisting of alternating layers of sintered electrode and dielectricwhich is finally leaded with end terminations to form the finishedcapacitor. Suspensions used for dielectric compositions in the past haveused both aqueous and organic liquids, but because of the environmentaland safety concerns, the tendency has been to increase the use ofaqueous suspensions for making the dielectric layers.

[0004] Another trend in the capacitor industry has been to make thedielectric layers thinner to obtain more capacitance per unit volume.Therefore, the thickness of dielectric layers have been reduced from 25microns to 10 microns. It is now desirable to reduce the thickness to 5microns or less. These thinner layers necessitate the use of extremelysmall solid ceramic particles in the suspension to produce the requiredhigh density and fine grain size in the final fired layer. When ceramicpowders are reduced to such small particle sizes, i.e., less than 0.5microns, they tend to have a significant soluble portion that dissolvesin an aqueous suspension thus causing chemical reactions with thedispersants and binders in solution.

[0005] Smaller particles are also more difficult to handle makingautomated systems unduly complex and expensive.

[0006] Barium titanate, the base material of choice for capacitorformulations due to its dielectric characteristics, forms a solublecation. Since the binder contains dispersing agents, any reaction of thesoluble cation or its companion hydroxyl ion with the chemicaldispersants in the binder can cause agglomerates of binder and “saltingout” or precipitation of the metal cation-dispersant complex. Thesecomplexes or agglomerates often create voids in the ceramic body duringthe binder burnout phase prior to sintering and can result in eitherelevated levels of electrical leakage or electrical shorting paths. Voidformation is particularly unforgiving in layers having a thickness ofless than 10 microns.

[0007] Another problem that ,occurs when making suspensions with ceramicpowder of less than 0.5 microns in diameter is that both the interfacialarea between the solids and the liquid carrier and the number ofparticles in a given volume are greatly increased. This results in ahigh physical chemical interaction between the solid particles in theliquid phase, and diminished processability, especially at commerciallyacceptable solids loading levels. Hence it can be expected that thebenefit of finer particle sizes can be countered by the necessity ofgoing to lower solids loadings in the suspensions or slips.Manufacturing processes which expose the suspension to high shearconditions such as those encountered in pumping or tape casting, resultin excessive gelling and in the worst case, dilatant-like conditionswhich are characterized by unworkable suspensions with shear thickeningcharacteristics and high viscosities.

[0008] A variety of attempts have been made to prepare finely dividedceramic powders in aqueous suspensions and slips. For example, U.S. Pat.No. 3,496,008 discloses the ball milling of a ferroelectric materialsuch as barium titanate in a 60% by weight solids loading level ofmilled material to water. The mixed suspension is rediluted to adesirable consistency for spray application.

[0009] In U.S. Pat. No. 3,551,197 a dielectric composition is preparedwith between 40 to 90 weight percent of a ceramic powder in water. Theceramic powder is selected from a group including barium titanate,strontium titanate, calcium titanate, and lead titanate, and has aparticle size of 0.5 to 3 micron. The suspended ceramic material iscombined with a binder such as polymethylene glycol or diethylene glycolfor example.

[0010] In U.S. Pat. No. 4,968,460, an aqueous emulsion of water solublepolymeric binder is combined with an aqueous suspension of ceramicmaterial in a solids loading of at least 50 weight percent. Thepolymeric binder is used in a range of 0.5 to 35 weight percent andoptionally with up to 5 weight percent of a selected dispersing agent.Tapes prepared from the slip composition had a thickness of between 30microns and 2.540 mm. Particle sizes in the range of 0.5 to 12 micronare disclosed.

[0011] These references however, do not address the problems encounteredin the preparation of aqueous suspensions or slips of ceramic powdershaving particles of less than 0.5 micron in diameter.

[0012] A suspension of ceramic powder having a diameter of 0.5 micron orless which remains suspended in an aqueous carrier fluid for extendedperiods of time in a substantially unagglomerated state and whichmaintains an apparent viscosity of less than 3000 centipoise (cps)without solidifying when determined from high shear rates of between 50to 100/ sec, would be a desirable improvement in the art of ceramicsuspensions, slips, and the processes for producing them.

[0013] Yet another object of the present invention is to produce anaqueous suspension which has a ceramic powder loading of up to about 30%by volume and has an apparent viscosity of less than 3000 cps.

[0014] Yet another objective of this invention is to passivate thesurface of the particle making up the ceramic powder with respect tosoluble anions and cations, with a very thin layer of a relativelyinsoluble passivating agent, such as barium oxalate, then apply adispersing agent to create a stable suspension. Surface passivation isnecessary to prevent interactions of the barium, or other ions, whichcan cause cross-linking with the dispersing agent(s).

[0015] An additional objective of the invention is to enable theformation of stable slips at high solids loading of up to about 30% byvolume of total solids (70 weight percent) or more of barium titanate,which have low enough viscosities to give good flow properties necessaryto make thin green layers; which contain the necessary binder to form acohesive film; and which have a uniform distribution of ceramicparticles.

[0016] A further object of the present invention is to provide a slipwhich is non-dilatant in high shear applications such as pumping andspraying.

SUMMARY OF THE INVENTION

[0017] Accordingly, the present invention provides a method forpreparing a ceramic composition by suspending a quantity of ceramicpowder, having an average particle size range of 0.5 micron or less in arange of up to 30% by volume of total solids in a carrier fluid composedof water and a quantity of passivating agent ranging from 0.5 to 5% byweight of ceramic powder. A quantity of either anionic or cationicdispersant is added to the suspended material in a range of at least 1%by weight based on the ceramic powder. The suspension and dispersingagent are then diluted with water and exhibit an apparent viscosity inthe range of less than 3000 centipoise.

[0018] In an alternate embodiment, a quantity of an organic binder isadded to the suspension in a range of 12 weight percent or less andpreferably 3 to 12 weight percent to form a slip composition. The slipmay then be applied to a substrate or mold to form a green layer havingpore sizes of less than 0.5 microns.

[0019] The features and advantages of the present invention are meant tobe illustrative rather than exhaustive. Further advantages and featuresof the present invention will become apparent while reviewing thedetailed description of the invention.

DETAILED DESCRIPTION OF THE INVENTION

[0020] To form a thin dense layer of a ceramic or dielectric powder,such as BaTiO₃ from an aqueous suspension of the ceramic powder, it isnecessary to create a stable suspension which resists settling at highsolids loading; which is processable, i.e. has an apparent viscositylevel which is sufficiently low to allow casting or pouring of thinfilms; has a Bingham yield point which provides that the applied layerof suspension achieves a gel state but which does not become dilatant;and where desired, contains an organic binder that will hold theparticles together during the subsequent operations, in the processprior to the ultimate high temperature sintering operation.

[0021] An additional characteristic of the present invention is toprovide a very thin coating on the particles which will produce a zetapotential which weakens the inherent interparticle attraction yet whichwill be more hydrophobic in nature, and will aid with the organic binderto control the physical properties of the green layer.

[0022] The inventors have discovered that for the preparation ofsuspensions and slips containing elevated ceramic powder loading levelsin aqueous carrier fluids, there exists a critical concentration rangefor passivating agents which will result in reduced agglomeration andmore uniform pore size distributions in the green layers and greenbodies made therefrom. In one embodiment, ceramic compositions for tapefabrication were prepared by uniformly suspending ceramic powder up toabout 30 volume percent and more particularly from 20 to 30 volumepercent in an aqueous carrier fluid of deionized water to which thepassivating agent had been previously added. The term “uniform” isdefined to mean pore sizes of 0.5 microns or less formed in green layerswhich were prepared from suspensions or slips of the present invention.Prepared suspensions had consistencies of from fluid-like to paste-likedepending on the loading level of ceramic powder. The dispersant wassubsequently added to the mixture to obtain a uniform suspension. Whilea variety of ceramic powders will be known to those skilled in the art,the powders that will benefit most from the passivation-dispersiontechnology are those powders that display phase instability in thepresence of water or those powders with relatively high solubilities orleachability of at least one of the metal components of the ceramicpowder between 10⁻⁴ M to 10 ⁻¹ M in a pH range of 4 to about 11.Furthermore, powders with an average particle size of 0.5 micron or lessand preferably between 0.05 micron and 0.5 micron (and having at leastone metal component in the solubility or leachability range noted above)will benefit most from the passivation-dispersion technology disclosedherein.

[0023] There are a number of methods for measuring average particle sizeof ceramic powders that are widely known to those skilled in the art. Inthe present invention, average particle size was determined by BET gasadsorption surface area analysis. This method is particularly useful incalculating average particle size when the powder particles aresubstantially spherical as is the case of the powders used in thepresent invention.

[0024] The term ceramic powders are further defined to include metallicoxides such as zinc oxide, bismuth oxide, or aluminum oxide; metallicsulfides, metallic borides, metallic nitrides, metallic carbides,metallic tellurides, metallic arsenides, metallic silicides, metallicselenides, and metallic halides; and including mixed materials such asmetallic titanates, metallic tantalates, metallic zirconates, metallicsilicates, metallic germanates, and metallic niobates.

[0025] The metal component of the metallic oxides may include thosemetals of the periodic table of elements found in groups IIA throughIIB, and also including the Lanthanum and Actinium series.

[0026] Ceramic powders are further defined to include complex oxideshaving the general formula ABO₃ wherein A consists of one or more of themetallic species which are of a similar ionic radius and ionic chargeand are selected from the group consisting of barium, calcium,magnesium, lead, strontium, and zinc. Group B consists of one or moremetallic species selected from the group consisting of hafnium, tin,titanium, zirconium, and may further include mixtures or solid solutionsthereof. Those skilled in the art will recognize the similarity of thecited B species with titanate based on reported values for ionic radiusand ionic charge.

[0027] In an alternate embodiment, ceramic powders are further definedto include the above identified complex oxides having the generalformula ABO₃, and may also contain one or more dopants. As the quantityof dopants to be added is normally a small weight percent of the totalsolids, the addition of a dopant generally does not affect the physicalcharacteristics of the slip or suspension. Those skilled in the art willtherefore recognize that a variety of “dopants” may be used. The term“dopants” shall be defined to include an additive which is used totailor the electrical performance of the ceramic powder in the finishedcapacitor. In the present invention, dopants may be defined to includeone or more metals selected from the oxides of the group consisting ofaluminum, antimony, bismuth, boron, calcium, cadmium, chromium, copper,cobalt, hafnium, iron, lanthanum, lead, manganese, molybdenum,neodymium, nickel, niobium, praseodymium, samarium, scandium, silicon,silver, tantalum, titanium, tin, tungsten, vanadium, yttrium, zinc, andzirconium.

[0028] The passivating agent can be any. acid or base which (i) burnsout cleanly from the suspension or slip at a temperature of about 1050°C. or less, (ii) provides a uniform surface charge on the ceramicparticles as a function of the carrier fluid pH, (iii) has reasonablyflat solubility over a pH range of 4 to about 11, (iv) forms relativelyinsoluble precipitates with at least 1 metallic species of the ceramicpowder, (v) promotes adsorption of desired anionic or cationicdispersants, and (vi) after adsorption of the dispetsant, has “improvedsettling characteristics”, defined hereafter to mean a sol which remainsopaque for a period of one week without appreciable precipation whencompared to control samples prepared in an aqueous carrier, at the samesolids loading but without a passivating agent. Analytical methods fordetermining the opacity of the sol are known to those skilled in the artand will not be elaborated upon herein.

[0029] While a number of passivating agents will be known to thoseskilled in the art, particularly desirable agents may include compoundsor mixtures of succinates, benzoates, formates, cupferons, and8-hydroxyquinoline. Although not wishing to be so limited, oxalic acidis the preferred passivating agent as discussed and evaluated in theExamples presented below. Preferably the oxalic acid is dissolved indeionized water at 1 to 3% by weight of ceramic powder.

[0030] The inventors have also discovered that for a ceramic powderloading level of up to 30 volume percent, an optimum passivating agentloading level of 1 to 3 weight percent exists for a quantity ofdispersing agent of greater than or equal to 1 weight percent. At lowerconcentrations of passivating agent, the ceramic powder begins to formagglomerates. At higher concentrations, the excess passivating agent inthe carrier fluid forms precipitates with dilute metals and dispersantsand apparent viscosity increases to unacceptably high levels, i.e. togreater than 3000 centipoise as determined at 50 to 100/s. The term“apparent viscosity” is known to those skilled in the art and shall bedefined hereafter to mean a value determined by fitting a power functionto a viscosity-shear rate curve which is prepared from actual viscositymeasurements. The resulting function is used to calculate the viscosityat shear rates of 50 sec⁻¹ and 100 sec⁻¹. A point-slope method is thenused to extrapolate to zero shear rate using the linear fit to theviscosity-shear rate curves to determine the apparent viscosity.

[0031] The present invention contemplates that dispersant concentrationsof greater than 5% by weight may be used depending on the passivatingagent and binder selected for a given slip composition.

[0032] While not wishing to be so limited, dispersing agents preferablyinclude the characteristics of being polymers which are; (i) compatiblewith a ceramic powder particle passivated in the manner discussed aboveand which uniformly coat the particle; (ii) which have trains whichstretch in a generally parallel manner across the particle surfacerather than extending radially therefrom; (iii) which minimizecrosslinking or “salting out” in the bulk suspension solution; and (vi)and which have a zeta potential in excess of +10 millivolts andpreferably either in the range of +10 millivolts to about +40 millivoltsor of −10 millivolts to about −40 millivolts for the loading levelsdiscussed above. A zeta potential magnitude which is less than ±10millivolts results in a suspension where there is insufficientelectrostatic repulsion to prevent particle agglomeration.

[0033] Various anionic and cationic surfactants having molecular weightin the range from less than 1000 to greater than 30,000 are contemplatedas dispersants. Included are sodium, potassium, or preferably ammoniasalts of stearate, lauryl sulfate, alkyl polyphosphate, dodecyl benzenesulfonate, disopropylnaphthalene sulfonate, dioctylsulfosuccinate,ethoxylated and sulfated lauryl alcohol, and ethoxylated and sulfatedalkyl phenol.

[0034] Various cationic surfactants include polyethyleneimine,ethoxylated fatty amine and stearylbenzyldimethylammonium chloride ornitrate. Alternate dispersants contemplated in the present inventioninclude: polyethylene glycols, lecithin, polyvinyl pyrrolidone,polyoxyethylene, isoctylphenyl ether, polyoxyethylene nonylphenyl ether,amine salts of alkylaryl sulfonates, polyacrylate and related salts,polymethacrylate and related salts, and fish oil. Additional anionic andcationic dispersants having the characteristics described above may befound in the reference entitled McCutcheon's, Volumes 1 and 2,McCutcheon Division, published by The Manufacturing ConfectionerPublishing Co.

[0035] In operation, up to 30 volume percent of a ceramic powder wassuspended in an aqueous carrier fluid containing from 0.5 to 5 weightpercent of passivating agent. A quantity of dispersing agent of greaterthan or equal to 1% by weight of the ceramic powder was then added tothe passivating agent containing carrier fluid. An additional quantityof water was then introduced to achieve the stated weight percentloading level.

[0036] A final suspension pH of between about 4 and about 11 andpreferably between 7 and 10 was achieved. Suspensions prepared by thismethod had Bingham yield points of less than 230 dynes/cm² and apparentviscosities of less than 3000 cps.

[0037] The inventors have discovered that the step of adding passivatingagent to the carrier fluid prior to the addition of the ceramic powderis critical to the performance of the suspension or slip as a greenlayer. While not wishing to be limited to any specific theory, it isconsidered that the dissolution reactions at the powder surface occurrapidly which requires the passivating agent to be available uponintroduction of the particles into the carrier media. If the passivatingagent is added after the powder is introduced, the precipitation verylikely occurs in the bulk solution rather than at the particle surface.

[0038] In an alternate embodiment, a quantity of dispersant may be addeddirectly to the carrier fluid with the passivating agent prior to theaddition of the ceramic powder.

[0039] In a method for producing a slip of the ceramic powder, aquantity of organic binder of 12% or less was added to the suspensionprepared according to the process discussed above and diluted furtherwith water.

[0040] The criteria for selecting a binder are that the binder mustuniformly disperse throughout the suspension and bind the passivated ordispersant coated ceramic particle while minimizing separation ofphases. The uniform distribution of passivated particles facilitatesparticle coating by the binder thereby allowing less binder to be used.Although not wishing to be so limited, polyethylene glycol, commerciallyavailable under the trademark “Carbowax ^(R)” was utilized. Carbowax isa trademark of Union Carbide of Danbury, Conn. In another embodiment,polyvinylpyrrolidone was used as a binder at 12% loading levels byweight of the ceramic material. In yet another embodiment a total of 13%of polyethyleneimine was used as a combined dispersant and binder.

[0041] Slip materials prepared at the loading levels discussed abovewere then applied on a substrate by methods known in the art to formgreen layers having a thickness of 5 microns or less.

[0042] By sandwiching conductive layers between multiple green layers,green bodies may be prepared. Those skilled in the art will be familiarwith the techniques for preparing green bodies and therefore suchtechniques will not be further discussed.

[0043] Referring to FIGS. 1A and B, scanning electron photo micrographsmade from suspensions prepared in accordance with Example 1, areillustrated at two different magnifications. Loading rates of 0.5 weightpercent of oxalic acid and 1 weight percent polyethyleneimine were used.As illustrated, agglomerated areas are clearly visible. Considerableimprovement in the degree of agglomeration is green in the micrographsof FIGS. 2 and 3, A and B, where green layers were prepared inaccordance with the procedures of Examples 2 and 3. Loading rates of 0.5weight percent oxalic acid at 5 weight percent of polyethyleneimine and1 weight percent oxalic acid and 1 weight percent of polyethyleneimine,were used. FIGS. 4 A and B are micrographs of cast green layers preparedin accordance with Example 5 where 1 weight percent of passivating agentand 3 weight percent of polyethyleneimine were used. When the amount ofpolyethyleneimine was increased to 5 weight percent, green layersillustrated in FIGS. 6 A and B were obtained which are similar to FIG.5. Where levels of oxalic acid from 2 to 3% were used and levels ofpolyethyleneimine from 1 to 5%, were used (FIGS. 7 to 9), commerciallyacceptable green layers were obtained. However, the micrographs of FIG.1, illustrate that where oxalic acid loading levels of 0.5 weightpercent were used at solids loading up to 30% by volume, and 1 weightpercent of polyethyleneimine was used, excessive agglomeration andunworkable viscosity was obtained. The micrographs demonstrate thecriticality of the lower range of passivating loading for aqueoussuspensions. While not wishing to be bound by any particular theory, oneexplanation is that the passivating effect of the oxalic acid isinsufficient at 0.5 weight percent to passivate the barium titanate atthe indicated level of solids loading.

[0044] Referring now to FIG. 10, aqueous slips were prepared inaccordance with the procedure described in Examples 16 and 17 where 3weight percent of oxalic acid and 1 weight percent of polyethyleneiminewere combined with 3 weight percent of polyethylene glycol binder. Themicrographs illustrated in FIGS. 10A and B show that the surface of thegreen layer has a uniform pore size distribution of 0.5 micron or less.

[0045] In FIG. 11, an aqueous slip was prepared in accordance with theprocedure described in Example 18 where 3 weight percent of oxalic acid,1 weight percent of polyethyleneimine was combined with 6 weight percentof polyethylene glycol binder. Commercially acceptable green layers wereobtained having a pore sizes 0.5 micron or less.

[0046] In FIG. 12, an aqueous slip was prepared in accordance with theprocedure described in Example 19 where 3 weight percent of oxalic acid,1 weight percent of polyethyleneimine was combined with 12 weightpercent of polyvinyl pyrroilidone binder. Commercially acceptable greenlayers were obtained having a pore size of less than 0.5 micron.

[0047] In FIG. 13, an aqueous slip was prepared in accordance with theprocedure described in Example 20 where 3 weight percent of oxalic acidand 2 weight percent of polyethyleneimine was combined with 11 weightpercent of polyvinyl pyrroilidone binder. An apparent viscosity of 1050cps was obtained.

[0048] In FIG. 14, an aqueous slip was prepared in accordance with theprocedure described in Example 21 where 3 weight percent of oxalic acidand 1 weight percent of polyethylen-eimine was added as a dispersant. Anadditional 12 weight percent of polyethyleneimine was added as thebinder. An apparent viscosity of 779 cps was obtained. The green layerformed had a pore size of 0.5 micron or less.

[0049] The present invention will become more readily understood by thefollowing non-limiting Examples. Apparent viscosity and loading levelsfor Examples 1-21 are summarized in Table 1 hereunder. Percentages arein weight percent of ceramic powder unless otherwise designated.

EXAMPLE 1

[0050] An aqueous solution of oxalic acid was prepared with 0.2810 goxalic acid (as H₂C₂O₄.2H₂O) and 23.2 g H₂0. To the oxalic acid solution56.8068 g of barium titanate, BT-10, produced by the assignee of thepresent invention, Cabot Corporation, having an average particle size ofabout 0.1 micron was slowly added with high shear mixing (normally 5000rpm). About 1.1433 g of polyethyleneimine or “PEI”, made by Kodak, 50%in water is added to the oxalic acid/barium titanate suspension at thesame mixing shear rate as above. The final suspension contained 0.5%oxalic acid and 1% PEI. The suspension obtained had an unacceptably highapparent viscosity in excess of 3000 cps and was therefore unsuitablefor the preparation of green ceramic layers by wet lay-down techniques.

[0051] Actual viscosity measurements were performed on the suspensionsat 25° C. using a cone-plate viscometer at shear rates ranging from 0.6to 120 sec⁻¹. If the measured viscosity of the suspensions was beyondthe range permitted by the measuring head of the viscometer or ifinsufficient data points were obtained to observe a plateau regime forthe viscosity-shear rate plot, the suspension viscosity was reported as“NA”. The cone plate viscometer was a Digital Viscometer, either model#LVTDCP or model DV-III Rheometer made by Brookfield EngineeringLaboratory, Inc. of Stoughton, Mass.

[0052] All suspensions for which heological data could be obtained werepseudoplastic. Many of the suspensions also exhibited a Bingham yieldstress, below which the slurry did not flow. This latter property isdesirable for most tape or layer forming processes to ensure that theslurry does not flow off of the substrate after deposition. Suspensionswhich were undergoing actual viscosity measurements were evaluated alsofor Bingham yield by recording the shear stress at a point where thesuspension or slurries tested exhibited no flow using a minimum shearrate equal to 0.6 sec⁻¹.

[0053] Zeta Potential Test Procedure

[0054] A quantity of suspension or slip prepared in accordance with theExamples was measured for zeta potential on a Brookhaven ZetaPlus,produced by Brookhaven Instruments Corporation, in Holtsville, N.Y.

[0055] Preparation of Green Layers

[0056] Green layers of the suspension or slip were prepared by thefollowing method:

[0057] About 2-3cc of the suspension or slip was added onto a glass sideand manually dispersed by means of running a metal blade run over thematerial to obtain uniform thickness. In another method, a manuallyoperated doctor blade was run over the deposited suspension and slipmaterials. The suspension was then dried for 10 to 15 minutes at roomtemperature to form the green layer.

[0058] EXAMPLES 2-15 were prepared in accordance with the procedure ofEXAMPLE 1 employing the quantities of reactants as indicated in Table 1.Percent by weight of oxalic acid as passivating agent andpolyethyleneimine as dispersant obtained in the final suspension, aswell as apparent viscosity values are presented in Table 1.

EXAMPLE 16

[0059] An aqueous solution of oxalic acid was prepared that contained1.7068 g oxalic acid (as H₂C₂O₄.2H₂O) to 20.0468 g H₂0. To the oxalicacid solution 56:8370 g of Cabot B-10 barium titanate is slowly addedwith high shear mixing (nominally 5000 rpm). Polyethyleneimine (PEI madeby Kodak, 50% in water) at 1.1349 g was added to the oxalic acid/bariumtitanate suspension under the same mixing shear as discussed above. Then1.6778 g of binder (Carbowax PEG 1450F, Union Carbide) was added to theexisting suspension. The final slip contained 3% oxalic acid, 1% PEI,and 3% binder as compared to the total weight percent of 69.82 (28.63volume percent) for the barium titanate powder. The slip was shearthinning with an apparent viscosity of 428 cps. Green layers wereprepared in accordance with the procedure of EXAMPLE 1.

[0060] EXAMPLE 17-21 were prepared in accordance with the procedure ofEXAMPLE 16 employing the quantities of reactants as indicated inTable 1. Percent by weight of oxalic acid as passivating agent anddispersant obtained in the final suspension, as well as apparentviscosity values are presented in Table 1. The dispersant utilized inEXAMPLE 19 and EXAMPLE 20 was polyvinyl pyrroilidone, available from GAFCorporation of Wayne, N.J. as PVP K-30. In EXAMPLE 21, polyethyleneimineat a concentration of 1 weight percent was used as the dispersant and 12weight percent was used as a binder. Green layers were produced by thedoctor blade procedure presented in Example 16. As is evident from Table1, both apparent viscosity and Bingham Yield Point values are inacceptable ranges for commercial applications. TABLE 1 Effect ofComposition on Viscosity Oxalic PEI Add Apparent Bingham Test Water AcidB-10 (50%) Water Binder % Solids % Solids Viscosity Yield Point # (g)(g) (g) (g) (g) (g) by Wt. by Vol. (cP) (Dynes/cm²) 1 23.2 0.2810(.5%)56.8068 1.1432(1%) 2.2074 67.92 26.02 > > — 2 25.33 0.2870(.5%) 56.80215.6799(5%) 64.5 23.2 1225 82 3 23.2 0.5682(1%) 56.8045 1.1410(1%) 2.159567.72 26.26 351 23 4 23.2 0.5705(1%) 56.8041 2.2712(2%) 2.1890 66.8025.43 911 57 5 23.2 0.5703(1%) 56.8048 3.4040(3%) 2.1592 65.94 24.34 50527 6 23.2 0.5675(1%) 56.8077 5.6835(5%) 2.1107 64.28 23.34 604 229 723.2 1.1372(2%) 56.8046 1.1491(1%) 2.1757 67.25 26.24 346 36 8 23.21.1310(2%) 56.8048 2.2728(2%) 2.1570 66.38 25.45 184 4.1 9 23.21.1350(2%) 56.8096 3.4051(3%) 2.1735 65.50 24.69 257 7.1 10 23.21.1358(2%) 56.8073 5.6989(5%) 2.1984 63.70 23.28 596 25 11 23.21.7036(3%) 56.8032 1.1565(1%) 2.1595 66.81 25.05 72 2.8 12 23.21.7087(3%) 56.8017 2.2697(2%) 2.1917 63.92 25.43 397 17 13 23.21.7043(3%) 56.8010 3.4078(3%) 2.1706 65.07 24.69 471 17 14 23.21.7089(3%) 56.8064 5.6793(5%) 2.1634 63.43 23.31 826 33 15 25.34792.8447(5%) 56.8069 5.6883(5%) 62.6 22.8 144 15 16 20.0468 1.7068(3%)56.8370 1.1349(1%) 0.4335 1.6778(3% PEG) 69.82 28.63 428 38 17 20.00281.7005(3%) 56.8021 1.1307(1%) 0.4319 1.6602(3% PEG) 69.87 28.68 412 3418 20.0111 1.7017(3%) 56.8039 1.1326(1%) 0.8986 3.3092(6% PEG) 68.4727.30 602 13 19 12.90 0.7487(3%) 24.9796 0.5000(1%) 0.4646 3.0069(12%PVP) 59.94 20.35 1600 210 20 12.14 0.7513(3%) 25.0017 1.0021(2%) 0.71592.7540(11%) 60.02 20.41 1050 80.6 21 9.48 0.7345(3%) 25.0213 6.5052(13%)3.4674 N/A 59.91 17.72 779 8.1

what is claimed is:
 1. A ceramic composition comprising: a quantity ofceramic powder uniformly suspended in an aqueous carrier fluid rangingup to 30% by volume of total solids in said suspension, said ceramicpowder having an average particle size of 0.5 micron or less, saidcarrier fluid having a quantity of passivating agent ranging from 0.5 to5% by weight of said ceramic powder, a quantity of dispersing agent ofat least 1% by weight, said suspension having an apparent viscosity ofless than 3000 cps.
 2. The composition of claim 1 wherein said ceramicpowder is a metal oxide having a metal component solubility in saidcarrier fluid of between 10 ⁻⁴ to 10 ⁻¹ moles per liter.
 3. Thecomposition of claim 2 wherein said suspension has a final pH of betweenabout 4 and
 11. 4. The composition of claim 3 wherein said ceramicpowder is a complex metal oxide having the formula of ABO₃, wherein A isat least one metallic species selected from the group consisting ofbarium, calcium, magnesium, lead, strontium, and zinc, and wherein B isat least one metallic species selected from the group consisting ofhafnium, tin, titanium and zirconium, or mixtures or solid solutionsthereof.
 5. The composition of claim 3 wherein said ceramic powder isaluminum nitride or silicon nitride.
 6. The composition of claim 3wherein said ceramic powder is zinc oxide, bismuth oxide, or aluminumoxide.
 7. The composition of claim 3 wherein said ceramic powder issuspended in said carrier fluid in an amount between 23% and 30% byvolume of total solids present in said suspension.
 8. The composition ofclaim 4 wherein said complex metal oxide includes one or more dopants.9. The composition of claim 4 wherein said ceramic powder is bariumtitanate.
 10. The composition of claim 4 wherein the ceramic powder is amixture of barium titanate and strontium titanate.
 11. The compositionof claim 4 wherein the ceramic powder is a mixture of lead titanate andzirconium titanate.
 12. The composition of claim 7 wherein saidpassivating agent is oxalic acid.
 13. The composition of claim 12wherein said dispersing agent uniformly coats said ceramic particles.14. The composition of claim 13 wherein said dispersing agent ispolyethyleneimine.
 15. The composition of claim 14 wherein said ceramicparticles, having been passivated and dispersant coated in said carrierfluids have a zeta potential magnitude between 10 millivolts and atleast +40 millivolts.
 16. The composition of claim 14 wherein saidsuspension has a Bingham yield point of up to 230 dynes/cm³.
 17. Thecomposition of claim 15 wherein said ceramic particles, having beenpassivated and dispersant coated in said carrier fluid, have a zetapotential magnitude between −10 millivolts and at least −40 millivolts.18. A ceramic suspension composition comprising: a quantity of a metaltitanate powder uniformly suspended in an aqueous carrier fluid rangingup to 30% by volume of total solids in said suspension and having anaverage particle size of 0.05 micron to 0.5 micron, said carrier fluidhaving a quantity of oxalic acid ranging from about 1 to about 3% byweight of said metal titanate powder, a quantity of polyethyleneimine ofat least 1% by weight of said metal titanate powder, said suspensionhaving an apparent viscosity of less than 3000 cps.
 19. The compositionof claim 18 wherein said quantity of oxalic acid is about 1% by weightof said metal titanate powder and said quantity of polyethyleneimine isabout 0.5% of said metal titanate powder.
 20. The composition of claim18 wherein said quantity of oxalic acid is about 5% and said quantity ofpolyethyleneimine is about 5% by weight of said metal titanate powder.21. The composition of claim 18 wherein said metal titanate powder is acomplex metal oxide having the formula of ABO₃, wherein A is at leastone metallic species selected from the group consisting of barium,calcium, magnesium, lead, strontium, and zinc, and wherein B istitanium.
 22. The composition of claim 18 wherein said complex metaloxide includes one or more dopants selected from the group consisting ofaluminum, bismuth, boron, manganese, neodymium, nickel, niobium,praseodymium, samarium, scandium, silver, tantalum, vanadium, andyttrium.
 23. The composition of claim 18 wherein said complex metaloxides is barium titanate in said carrier fluid.
 24. The composition ofclaim 18 wherein said particles of complex metal oxides have a surfacezeta potential between +10 millivolts and at least +40 millivolts. 25.The composition of claim 18 wherein said particles of complex metaloxides have a surface zeta potential between −10 millivolts and at least−40 millivolts in said carrier fluid.
 26. The composition of claim 18wherein said suspension has a Bingham yield point of up to 230dynes/cm³.
 27. A ceramic slip composition comprising: quantity ofceramic powder uniformly suspended in an aqueous carrier fluid rangingup to 30% by volume of total solids in said suspension, said ceramicpowder having a particle size of 0.5 micron or less, said carrier fluidhaving a quantity of passivating agent ranging from 0.5 to 5% by weightof said ceramic powder, a quantity of dispersing agent of at least 1% byweight of said ceramic powder, a quantity of organic binder of 12% orless by weight of said ceramic powder, said slip having a Bingham yieldpoint of less than or equal to 210 dynes/cm² and an apparent viscosityof less than 3000 cps.
 28. The composition of claim 27 wherein saidceramic powder is a complex metal oxide having the formula of ABO₃,wherein A is at least one metallic species selected from the groupconsisting of barium, calcium, magnesium, lead, strontium, zinc, andwherein B is at least one metallic species selected from the groupconsisting of hafnium, tin, titanium and zirconium, or mixtures or solidsolutions thereof.
 29. The composition of claim 28 wherein said complexmetal oxide includes one or more dopants.
 30. The composition of claim28 wherein the complex metal oxide is a mixture of barium titanate andstrontium titanate.
 31. The composition of claim 28 wherein the ceramicpowder is a mixture of lead titanate and zirconium titanate.
 32. Thecomposition of claim 28 wherein said ceramic powder is barium titanate.31. The composition of claim 26 wherein said ceramic powder is zincoxide, bismuth oxide, or aluminum oxide.
 32. The composition of claim 30wherein said passivating agent is oxalic acid.
 33. The composition ofclaim 32 wherein said slip has a final pH of between about 7 and
 10. 34.The composition of claim 33 wherein said dispersing agent ispolyethyleneimine.
 35. The composition of claim 34 wherein said organicbinder is polyethylene glycol.
 36. The composition of claim 34 whereinsaid organic binder is polyvinylpyrrolidone.
 37. The composition ofclaim 34 wherein said organic binder is polyethyleneimine.
 38. Thecomposition of claim 35 wherein said slip has a Bingham yield point ofless than or equal to 210 dynes/cm².
 39. A method for preparing asuspension of ceramic powder comprising: mixing up to 30% by volume of aceramic powder having an average particle size of 0.05 micron to 0.5micron in an aqueous carrier fluid containing a quantity of passivatingagent ranging from about 0.5 to about 5% by weight of said ceramicpowder, mixing a quantity of a dispersing agent of at least 1% by weightof said ceramic powder with said suspension to achieve a uniformlydistributed suspension with improved stability, and introducing anadditional quantity of water to said suspension to achieve said volumepercent loading level, and an apparent viscosity of less than 3000 cps.40. The method of claim 39 wherein said ceramic powder is a complexmetal oxide having the formula of ABO₃, wherein A is at least onemetallic species selected from the group consisting of barium, calcium,magnesium, lead, strontium, zinc, and wherein B is at least one metallicspecies selected from the group consisting of hafnium, tin, titanium andzirconium, or mixtures or solid solutions thereof.
 41. The method ofclaim 39 wherein said complex metal oxide includes one or more dopants.42. The method of claim 39 wherein said ceramic powder is a mixture oflead titanate and zirconate titanate.
 43. The method of claim 39 whereinsaid ceramic powder is a mixture of barium titanate and strontiumtitanate.
 44. The method of claim 39 wherein said quantity of ceramicpowder is barium titanate.
 45. The method of claim 39 wherein saidceramic powder is zinc oxide, bismuth oxide, or aluminum oxide.
 46. Themethod of claim 39 wherein said passivating agent is a dilute aqueoussolution of oxalic acid.
 47. The method of claim 39 wherein saiddispersant is polyethyleneimine.
 48. The method of claim 39 wherein saidsuspension has a final pH of between about 7 and
 10. 49. The method ofclaim 39 wherein said particles of ceramic material have a surface zetapotential between +10 millivolts and at least +40 millivolts in saidcarrier fluid.
 50. The method of claim 39 wherein said particles ofceramic material have a surface zeta potential between −10 millivoltsand at least −40 millivolts in said carrier fluid.
 51. The method ofclaim 39 wherein a quantity of an organic binder is added to saidsuspension to form a slip composition.
 52. The method of claim 39wherein said suspension achieves a Bingham yield point of less than 230dynes/cm²
 53. A method for preparing a suspension of ceramic powder bythe steps consisting of: mixing up to 30% by volume of a ceramic powderhaving an average particle size of 0.05 micron to 0.5 micron in anaqueous carrier fluid containing a quantity of passivating agent rangingfrom 0.5 to 5% of said ceramic powder, mixing a quantity of a dispersingagent ranging from at least 1% by weight of said ceramic powder withsaid suspension to achieve a uniform distribution of said ceramic powderwith improved stability, and introducing an additional quantity of waterto achieve said ceramic powder loading level, and obtaining and anapparent viscosity of less than 3000 cps.
 54. The method of claim 53having a Bingham yield point of less than or equal to 210 dynes/cm². 55.A method for preparing a slip of ceramic powder by the steps consistingof: mixing up to 30% by volume of a ceramic powder having an averageparticle size of 0.05 micron to 0.5 micron in an aqueous carrier fluidcontaining a quantity of passivating agent ranging from 0.5 to 5% byweight of said ceramic powder, mixing a quantity of a dispersing agentranging from at least 1% by weight of said ceramic powder with saidsuspension to achieve a uniformly distributed suspension with improvedstability, adding a quantity of organic binder to form a slip, andintroducing an additional quantity of water to achieve said volumepercent loading level, and an apparent viscosity of less than 3000 cps.56. The method of claim 55 wherein said slip has a Bingham yield pointof less than or equal to 210 dynes/cm².
 57. The method of claim 56wherein said ceramic powder is a complex metal oxide having the formulaof ABO₃, wherein A is at least one metallic species selected from thegroup consisting of barium, calcium, magnesium, lead, strontium, zinc,and wherein B is at least one metallic species selected from the groupconsisting of hafnium, tin, titanium, zirconium, mixtures and solidsolutions thereof.
 58. The method of claim 56 wherein said complex metaloxide includes one or more dopants.
 59. The method of claim 56 whereinsaid ceramic powder is a mixture of barium titanate and strontiumtitanate.
 60. The method of claim 56 wherein said ceramic powder is amixture of lead titanate and zirconium titanate.
 61. The method of claim56 wherein said ceramic powder is barium titanate.
 62. The method ofclaim 56 wherein said ceramic powder is zinc, bismuth, or aluminumoxide.
 63. The method of claim 56 wherein said passivating agent isoxalic acid.
 64. The method of claim 63 wherein said dispersant ispolyethyleneimine.
 65. The method of claim 64 wherein said slip has afinal pH of between about 7 and
 10. 66. The method of claim 65 whereinsaid particles of ceramic material have a surface zeta potential between+10 millivolts and at least +40 millivolts in said carrier fluid. 67.The method of claim 65 wherein said particles of ceramic material have asurface zeta potential between −10 millivolts and at least −40millivolts in said carrier fluid.
 68. The method of claim 65, whereinsaid organic binder is polyethylene glycol.
 69. The method of claim 65,wherein said organic binder is polvinylpyrrolidone.
 70. The method ofclaim 65 wherein said organic binder is polyethyleneimine.
 71. Themethod of claim 68 wherein said binder is present in said slip in arange of from 3% to 6% by weight of the barium titanate suspended insaid slip.
 72. The method of claim 69 wherein said binder is present insaid slip in a range of up to 12% by weight of the barium titanatesuspended in said slip.
 73. The method of claim 70 wherein said binderis present in said slip in a range of up to 12% by weight of the bariumtitanate in said slip.
 74. A method for preparing a slip of ceramicpowder by the steps consisting of: mixing up to 30% by volume of aceramic powder having an average particle size of 0.05 micron to 0.5micron in an aqueous carrier fluid containing a quantity of oxalic acidranging from 0.5 to 5% by weight of said ceramic powder, mixing aquantity of a polyethyleneimine of at least 1% by weight of said ceramicpowder with said suspension to achieve a uniform distribution of saidceramic with improved stability, adding 12% of polyethyleneimine byweight ceramic powder to form a slip, and introducing an additionalquantity of water to achieve said volume percent loading level, andobtaining a Bingham yield point of 8 dynes/cm² and an apparent viscosityof 779 cps.
 75. A green layer prepared by the method of claim 73comprising applying the slip to a support means to a uniform thicknessand subjecting said applied slip to a temperature sufficient tovolatilize the aqueous component.